1. Field
This invention generally relates to oxygen generating devices and particularly to electrochemical oxygen generating devices using metal oxide, oxygen ion conducting electrolyte.
2. State of the Art
Electrochemical devices which employ oxygen ion conducting electrolytes are well known. Such devices are used as sensors whereby the oxygen partial pressure difference existing between opposite sides of said electrolyte create a voltage potential which may be determined to indicate a level of oxygen concentration on one side or the other, especially when the oxygen concentration on one side of the sensor is known. Such sensors are used in automobile engines, furnaces and other devices wherein it is desired to operate at stoichiometric ratios between the fuel and the air or oxygen necessary for combustion of fuel.
Also, such electrochemical devices, when operated in a current mode with an applied voltage may be utilized to generate pure oxygen. Devices of this type are discussed in certain patents to Ruka, for example, Re. 28,792.
Certain difficulties have generally been encountered with such oxygen sensors and oxygen generating devices. In electrochemical sensors it is common practice to utilize platinum as an electrode or to utilize various electrode layers, for example, a platinum electrode adjacent to the electrolyte with an overcoating of a protective porous film. Platinum has been generally employed because of its catalytic activity and because of its relatively high melting point among conductive metals. It has been found, however, that the use of platinum in oxygen generating electrochemical cells that the platinum, which is relatively conductive, has an apparent resistance higher than what would normally be expected. Thus, oxygen generating electrochemical cells utilizing platinum electrodes have been electrically inefficient. Furthermore, since the platinum electrodes must be porous in order to permit oxygen molecules to reach the surface of the electrolyte at the cathodes, and, upon recombination at the anode surface, to depart from the electrolyte. While pores are thus necessary, the effective electrode-electrolyte interface for electrical purposes is consequently reduced.
Sensors generally are quite small, frequently formed as a disk smaller than a dime or as a thin thimble having a length of about one-half inch and an outside diameter of less than one-fourth inch. The amount of voltage or current applied or produced by sensors is very small, generally being in the millivolt and milliamp range. The problems of uniform current distribution over a broad area is generally not encountered because of the small size of the device.
The aforementioned patents of Joshi describe many of the considerations involved in producing useful oxygen delivery devices and further describe certain advantageous electrode/electrolyte systems.
Sensors are produced to maximize response time, to endure repeated hot/cold cycling, to be reliable over an extended period of time. Maximizing the quantity of oxygen transported through the electrolyte per amp applied is generally not a factor in sensor design or fabrication.
An oxygen delivery device, while employing oxygen conducting electrolytes and current carrying electrodes, has different objectives than a sensor and involves different considerations. An oxygen delivery device employing larger electrolytes has a very large surface area in comparison to a sensor. Because of the size of the electrolyte, strength is an important factor. Also, the problems of differential temperatures may create stress problems, especially if an area of an electrolyte begins conducting more oxygen ions than other areas, which results in hot spots. A hot spot may be a result of uneven distribution of current by the electrode or a thin wall spot on the electrolyte.
While platinum has generally been the standard electrode for zirconia-type sensors, its use in oxygen delivery devices has been generally unsatisfactory.